[llvm] [BOLT][AArch64] Transform cmpbr ~> cmp + br when inversion not possible (PR #185731)

[Paschalis Mpeis via llvm-commits]

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@@ -318,6 +321,123 @@ TEST_P(MCPlusBuilderTester, AArch64_ReverseCompAndBranch) {
   ASSERT_FALSE(BC->MIB->isReversibleBranch(Overflows));
 }
 
+TEST_P(MCPlusBuilderTester, AArch64_ReverseCompAndBranch_Underflows) {
+  if (GetParam() != Triple::aarch64)
+    GTEST_SKIP();
+
+  BinaryFunction *BF = BC->createInjectedBinaryFunction("BF", true);
+  BinaryBasicBlock *EntryBB = BF->addBasicBlock();
+  BinaryBasicBlock *FallThroughBB = BF->addBasicBlock();
+  BinaryBasicBlock *TargetBB = BF->addBasicBlock();
+  BF->addEntryPoint(*EntryBB);
+  EntryBB->addSuccessor(TargetBB);
+  EntryBB->addSuccessor(FallThroughBB);
+
+  // Inversion requires expansion, immediate value underflows.
+  // cblt x0, #0, target ~> cmp x0, #0
+  //                        b.ge target
+  auto I =
+      EntryBB->addInstruction(MCInstBuilder(AArch64::CBLTXri)
+                                  .addReg(AArch64::X0)
+                                  .addImm(0)
+                                  .addExpr(MCSymbolRefExpr::create(
+                                      TargetBB->getLabel(), *BC->Ctx.get())));
+  BinaryFunctionCallGraph CG(buildCallGraph(*BC.get()));
+  RegAnalysis RA(*BC.get(), &BC->getBinaryFunctions(), &CG);
+  DataflowInfoManager DIM(*BF, &RA, nullptr);
+
+  ASSERT_TRUE(BC->MIB->isReversibleBranch(*I, &DIM));
+  BC->MIB->reverseBranchCondition(EntryBB, *I, TargetBB->getLabel(),
+                                  BC->Ctx.get());
+  auto R = EntryBB->rbegin();
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paschalis-mpeis wrote:

Any reason to `rbegin()` here?

https://github.com/llvm/llvm-project/pull/185731